1. Field of the Invention
The present invention relates to a developing method to develop an electrostatic latent image with a two component-type developer which contains a magnetic material dispersion-type resin carrier for electrophotography.
2. Related Background Art
Glass beads, iron powder, ferrite powder and fine particles of magnetic material dispersion type resins are conventionally known as carriers which are to be used in combination with toners to develop electrostatic latent images in electrophotography, and it is general in these days to use a carrier which comprises iron powder, ferrite powder and fine particles of magnetic material dispersion-type resin due to demands for developing processes.
For a two component-type developing method which uses a two component-type developer comprising a carrier and a toner as described above, there has been proposed to weaken a magnetic force of the carrier as a technique to enhance qualities of copied images. This proposal is an attempt to form a finer and softer magnetic brush on a developer carrying member by weakening a magnetic force of the carrier, thereby making it possible to form a high fidelity image without splashes or scatters of the toner in developing an electrostatic latent image on a photosensitive drum with the developer. Furthermore, use of the carrier which has a weakened magnetic force lessens deterioration of the developer during developing operations repeated to reproduce a large number of, copies, thereby making it possible to obtain high quality images for a long time.
As methods to obtain a carrier which has a weakened magnetic force, there can be mentioned one which prepares a carrier as ferrite by mixing an iron oxide with a non magnetic metallic oxide which is an impurity and another which prepares a carrier by dispersing a magnetic powder in a binder resin. Magnetic material dispersion-type resin carriers which contain dispersed magnetic materials are preferable in particular since the magnetic forces and particle sizes can easily be controlled. For this reason, carriers having uniform and small particle sizes which are prepared by polymerization methods or the like are used in these days where there are demands for toners and carriers which have smaller particle sizes.
When development is repeated to reproduce a large number of copies using such a resin carrier as a developer, however, inconvenience is conventionally encountered that image densitys are lowered as the developer is used for a longer time. Furthermore, the developer poses another problem that fog is produced on non-image areas when a toner is repeatedly replenished to keep a toner content constant in the developer. Making elaborate examinations of this phenomenon, the inventors of the present invention found that a fluidity of the developer was remarkably lowered as development was repeated to reproduce a large number of copies and that a cause for the lowering of the fluidity lay in residual magnetization of the carrier which was increased as the developer was used. Though a magnetic material which has a weak coercive force is used as a component of the magnetic material dispersion-type resin carrier, it is considered that the resin carrier is gradually magnetized on a developer carrying member as development was repeated, thereby resulting in enhancement of residual magnetization and constituting a cause for the lowering of the fluidity of the developer.
With regard to a conventionally known carrier which has residual magnetization, Japanese Patent Application Laid-Open No. 59-501840 made a proposal. The carrier described in this patent is a hard type magnetic carrier which has a coercive force H.sub.c of not weaker than 300 Oe when it is magnetically saturated, thereby allowing high magnetization to remain. The carrier is used for the purpose of achieving image stability, or of stabilizing density of images and preventing adhesion of the carrier, in a high-speed copying process with a rotating magnet core-type magnetic applicator, and it can be said that the carrier having the residual magnetization as described in the proposal mentioned above is used exclusively for a developing method for which the developing process with the rotating magnet core-type applicator or the like is indispensable.
Examinations made by the inventors indicated that slight residual magnetization is produced even in a carrier which has a coercive force H.sub.c lower than 300 Oe, and that the carrier in which the residual magnetization is produced is in rather a moniliform condition as shown in FIG. 1A even in an atmosphere free from a magnetic field and agglomerated gradually at higher degrees as development is repeated from a condition where the carrier is not magnetized (see FIGS. 1A through 1C). Furthermore, it could be observed that the phenomenon was remarkable in a resin carrier which had a small particle diameter in particular, and that paramagnetism was produced and changed with time when a carrier had a coercive force which was smaller than a maximum value of an intensity of a magnetic field on a developing magnetic pole. In contrast, it was found that the carrier could not be magnetized by the developing magnetic pole and a variation of residual magnetization did not cause the variation of fluidity with time described above when a coercive force was larger than the maximum value of the intensity of the magnetic field on the developing magnetic pole or zero. That is, the examinations made by the inventors clarified that residual magnetization is produced in a magnetic carrier having a coercive force which is not zero and smaller than the maximum value of the intensity of the magnetic field on the developing magnetic pole, thereby agglomerating the carrier at higher degrees as development is repeated and constituting a cause for degradation of carrier characteristics such as a fluidity.
Furthermore, it is considered that the content of toner in the developer at the time of reproduction of many copies is changed for the reason given below. As the fluidity of carrier is lowered, a replenished toner is poorly taken into the carrier, and as a result, a sensor which detects the toner content in the developer judges the amount of the toner to be excessive and functions to prevent the toner from being replenished to the carrier. It is considered that insufficiency of the toner which is taken into the carrier increases portions of the toner which are electrified insufficiently and reversely, thereby inversion component of charge appears as fog on the non-image areas. In the case when there occurs the lowering of the toner taken into the carrier, the increase of the portions of the toner which are electrified insufficiently and reversely was clarified by observing a transition of a distribution of electrified charge quantity.
However, there has been proposed no effective method to solve the problem that image densities are lowered when development is repeated to reproduce a large number of copies using a developer which contains a resin carrier as described above or a problem of the fog on the non-image areas which is produced when a toner is replenished repeatedly to keep the toner content in a developer at a constant level.